1. Field of the Invention
The invention relates to the field of the optical transfer of patterns by projecting and more particularly, relates to an illuminator for such optical transfer.
2. Description of the Prior Art
In the manufacture of integrated circuits, windows make it possible to locate the implantation or treatment on the substrate (silicon, gallium arsenide) for each stage in the manufacture of the integrated circuit level on the substrate. These windows are produced by masking a resin layer deposited on the substrate from a mask. According to an early method, the mask was placed in contact or in the vicinity of the substrate for exposure. The tendency has been to replace this method by an optical projection transfer method, which can be performed with a selected magnification and the pattern can be repeated by the photorepetition method.
For the optical transfer of patterns, it is possible to use illuminators which can, for example, be laser sources or arc discharge lamps, more particularly mercury discharge lamps. These are associated with suitable filters for obtaining an illumination which is compatible with the often vary narrow spectral band of the projection lenses (B.perspectiveto.100 mm). The light from the source illuminates the object (i.e. the mask) via condensers, the object being positioned in the vicinity of the the latter. In general terms, it has been found that a compromise between the effective resolution likely to be obtained and the field could be obtained by using a partly spatially coherent light. The ratio .sigma. between the diameter of the line projected onto the entrance pupil of the optical projection system and the diameter of the pupil constitutes a measure of the coherence of the illumination. When .sigma.=0, the illumination is coherent, when .sigma.=.infin., the illumination is incoherent and when .sigma.=1 the illumination is quasi-incoherent. In the case of illuminators associated with conventional projection lenses of numerical apperture 0.3 and diameter 15 mm for example, this compromise is at present .sigma.=0.7. In the case of such illuminators, account is taken of the energy efficiency, .sigma.=0.7 being a value for which this efficiency is considered suitable. In order to obtain such a coefficient .sigma., a suitably diaphragmed extended source is used.
Calculation has shown that a significant improvement in a certain number of parameters on which the performance ratings of the optical transfer device depend is obtained with a coefficient .sigma.=0.3, i.e. a more spatially coherent illumination than that used in a conventional illuminator in which .sigma. is 0.7. Thus, the effective resolution increases with the coherence. The width of the minimum line likely to be obtained decreases when the coherence of the illumination increases. The definition also increases when the coherence increases. Moreover, defects due to thickness variations in the resin layer deposited on an already shaped surface decrease when coherence increases. Taking into account of all these parameters, an increase in the coherence of the illumination is desirable to obtain finer and more accurately controlled patterns.
However, the energy efficiency must not become too low and the defects linked with an excessive coherence of the illumination must not appear. These disadvantages can lead on the one hand to an excessive limitation of the diameter of the effective source and on the other in a greater sensitivity of the transfer device to local defects (and to dust) present on the reticle and on the projection lens. These defects linked with a high degree of coherence with regard to surface defects can be limited by taking care of the projection lens.
The energy efficiency can be maintained in an illuminator chosen with .sigma.=0.3 by the optimum utilization of the intensity distribution supplied by the source in conjunction with the entrance pupil of the lens in order that the limitation to the dimensions of the source have minimum effect on the energy efficiency.